Forecasting the Frequency and Magnitude of Postfire Debris Flows Across Southern California

Kean, Jason W.; Staley, Dennis M.

doi:10.1029/2020ef001735

Cited by 50 publications

(52 citation statements)

References 97 publications

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“…intervals ranging from 1-1000 years, the 15-, 30-, and 60-min rainfall intensities for the Santa Cruz Mountains are, on average, 20.7% higher than the San Gabriel Mountains (NOAA, 2020). Kean and Staley (2021), who present a framework to forecast the frequency and magnitude of postwildfire debris flows, show that for the RCP4.5 emissions trajectory, where rainfall intensity is projected to increase in southern California by 18% (i.e., similar in magnitude to the 20.7% value reported above), the probability of at-threshold and major debris flow events should increase.…”

Section: Accepted Articlementioning

confidence: 88%

Postwildfire Soil‐Hydraulic Recovery and the Persistence of Debris Flow Hazards

et al. 2021

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Deadly and destructive debris flows often follow wildfire, but understanding of changes in the hazard potential with time since fire is poor. We develop a simulation‐based framework to quantify changes in the hydrologic triggering conditions for debris flows as postwildfire infiltration properties evolve through time. Our approach produces time‐varying rainfall intensity‐duration thresholds for runoff‐ and infiltration‐generated debris flows with physics‐based hydrologic simulations that are parameterized with widely available hydroclimatic, vegetation reflectance, and soil texture data. When we apply our thresholding protocol to a test case in the San Gabriel Mountains (California, USA), the results are consistent with existing regional empirical thresholds and rainstorms that caused runoff‐ and infiltration‐generated debris flows soon after and three years following a wildfire, respectively. We find that the hydrologic triggering mechanisms for the two observed debris flow types are coupled with the effects of fire on the soil saturated hydraulic conductivity. Specifically, the rainfall intensity needed to generate debris flows via runoff increases with time following wildfire while the rainfall duration needed to produce debris flows via subsurface pore‐water pressures decreases. We also find that variations in soil moisture, rainfall climatology, median grain size, and root reinforcement could impact the median annual probability of postwildfire debris flows. We conclude that a simulation‐based method for calculating rainfall thresholds is a tractable approach to improve situational awareness of debris flow hazard in the years following wildfire. Further development of our framework will be important to quantify postwildfire hazard levels in variable climates, vegetation types, and fire regimes.

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Section: Accepted Articlementioning

confidence: 88%

Postwildfire Soil‐Hydraulic Recovery and the Persistence of Debris Flow Hazards

et al. 2021

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“…However, one major limitation of using Atlas 14 for planning purposes is that it assumes a stationary climate-and it is clear that extreme precipitation events have already increased in response to climate change across continental North America (Kirchmeier-Young & Zhang, 2020). For resilient and sustainable resource management, it is critical that decision-makers utilize tools and information that account for the hydrologic changes anticipated in a warming climate (Milly et al, 2008 To assess the impacts of climate change on post-wildfire debris-flow frequency and magnitude, Kean and Staley (2021) apply the Clausius-Clapeyron (C-C) relation, which states that the amount of water vapor at saturation increases at a rate of 7%/°C. The authors use the first-order assumption, following from C-C, that rainfall intensifies at the same rate of water vapor in the atmosphere (Trenberth et al, 2003).…”

Section: Precipitation Intensification In a Warming Climatementioning

confidence: 99%

“…Given these circumstances, pre‐fire efforts to mitigate post‐fire hydrologic hazards may be more successful and feasible than rapid post‐fire response (Figure 1). The tool presented in Kean and Staley (2021) supports pre‐fire planning decisions such as where to target mitigation efforts, informing building codes and landscape practices, where to focus education of residents, or in developing targeted evacuation plans. This information can be incorporated into state, county, or local entity Hazard Mitigation Plans, which may already highlight post‐wildfire debris flows as a hazard (e.g., Ventura, 2015).…”

Section: Debris Flow Recurrence Interval As a Pre‐fire Decision Support Toolmentioning

confidence: 99%

“…Observations from one well‐documented region (e.g., Southern California) cannot necessarily be extrapolated to another due to unique weather, climate, ecology and geology. As calibrated debris‐flow models and thresholds are the basis of the decision support tool presented in Kean and Staley (2021) for Southern California, similar information is necessary to apply this tool in other debris flow‐susceptible regions. It is critical to capture the timing of post‐fire debris‐flow initiation and associated rainfall intensities.…”

Section: Monitoring Is Key For Expansion Of Decision Support Tools To Other Regionsmentioning

confidence: 99%

“…Decision-makers are in need of resources to support planning for the changing characteristics of these hazards. A novel framework for evaluating post-fire debris flow recurrence intervals (Kean & Staley, 2021,…”

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A Warming Climate Adds Complexity to Post‐Fire Hydrologic Hazard Planning

Oakley

2021

Earth's Future

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Wildfire removes vegetation and weakens root strength, making burned areas more susceptible to erosion. Wildfire can also alter the physical and chemical characteristics of soil, creating a hydrophobic layer which increases runoff and may lead to damaging floods and debris flows in steep terrain. For example, following the 2017 Thomas Fire in Southern California, the devastating 9 January 2018 debris flows in the community of Montecito resulted in 23 deaths and nearly $1B in damages (Lancaster et al., 2021). In a meeting following the event, a Santa Barbara County resource manager made a plea to researchers: "I need to know how frequently I can expect post-fire debris flows of this magnitude in this area." Managers in other susceptible areas have expressed similar concerns for events in their area of responsibility.Impactful post-fire debris flows are not new to southern California, though the penultimate fatality event occurred in 2003 when 16 people died in debris flows emanating from the Old/Grand Prix burn areas in the San Bernardino Mountains (Oakley et al., 2017). The Montecito event renewed focus on post-fire debris-flow preparedness, with understanding the recurrence intervals of these events in the current and future climate as key concerns in planning efforts. Southern California's well-documented history of impactful post-fire debris flows compared to other susceptible regions makes it an excellent "laboratory" for research and development of decision support tools. Kean and Staley (2021) utilize Southern California's data to provide the first known framework to estimate recurrence intervals for minor and major post-fire debris flows in the region. This tool allows for evaluation of recurrence intervals in both a stationary and warming climate. Emerging research from disciplines relevant to post-fire hazards (e.g., soil, wildfire, and climate science) can be readily integrated into this framework. While regionally focused, this tool is applicable to other locations susceptible to post-wildfire debris flows provided the requisite data on rainfall triggering thresholds, burn severity, and wildfire recurrence can be collected. Debris Flow Recurrence Interval as a Pre-Fire Decision Support ToolThe time between wildfire and precipitation events is decidedly brief in California, such that pre-fire planning presents significant advantages. For example, the 2018 Montecito debris flow occurred before the Thomas Fire was 100% contained (Lancaster et al., 2021). Observations over the past 60 years demonstrate a delayed onset of the rainy season in California (Lukovic et al., 2021). This allows dry conditions to extend into the autumn/winter offshore wind season (Abatzoglou et al., 2021;Swain, 2021), amplifying wildfire risk (Goss et al., 2020). Climate model projections suggest further "sharpening" of the precipitation Abstract Climate change will likely increase the frequency of damaging post-wildfire floods and debris flows, amplifying the threat to life, property, and infrastructure situated in susceptible are...

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The grain size of sediments delivered to steep debris‐flow prone channels prior to and following wildfire

Neely,

Moon,

DiBiase

et al. 2024

Earth Surf Processes Landf

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Debris flows are powered by sediment supplied from steep hillslopes where soils are often patchy and interrupted by bare‐bedrock cliffs. The role of patchy soils and cliffs in supplying sediment to channels remains unclear, particularly surrounding wildfire disturbances that heighten debris‐flow hazards by increasing sediment supply to channels. Here, we examine how variation in soil cover on hillslopes affects sediment sizes in channels surrounding the 2020 El Dorado wildfire, which burned debris‐flow prone slopes in the San Bernardino Mountains, California. We focus on six headwater catchments (<0.1 km2) where hillslope sources ranged from a continuous soil mantle to 95% bare‐bedrock cliffs. At each site, we measured sediment grain size distributions at the same channel locations before and immediately following the wildfire. We compared results to a mixing model that accounts for three distinct hillslope sediment sources distinguished by local slope thresholds. We find that channel sediment in fully soil‐mantled catchments reflects hillslope soils (D50 = 0.1–0.2 cm) both before and after the wildfire. In steeper catchments with cliffs, channel sediment is consistently coarse prior to fire (D50 = 6–32 cm) and reflects bedrock fracture spacing, despite cliffs representing anywhere from 5% to 95% of the sediment source area. Following the fire, channel sediment size reduces most (5‐ to 20‐fold) in catchments where hillslope sources are predominantly soil covered but with patches of cliffs. The abrupt fining of channel sediment is thought to facilitate postfire debris‐flow initiation, and our results imply that this effect is greatest where bare‐bedrock cliffs are present but not dominant. A patchwork of bare‐bedrock cliffs is common in steeplands where hillslopes respond to channel incision by landsliding. We show how local slope thresholds applied to such terrain aid in estimating sediment supply conditions before two destructive debris flows that eventually nucleated in these study catchments in 2022.

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Forecasting the Frequency and Magnitude of Postfire Debris Flows Across Southern California

Abstract: Wildfire substantially increases the susceptibility of steep slopes to debris flow, a fast-moving mixture of water, soil, and rock that can cause property damage and loss of life (e.g.

Cited by 50 publications

References 97 publications

Postwildfire Soil‐Hydraulic Recovery and the Persistence of Debris Flow Hazards

Postwildfire Soil‐Hydraulic Recovery and the Persistence of Debris Flow Hazards

A Warming Climate Adds Complexity to Post‐Fire Hydrologic Hazard Planning

The grain size of sediments delivered to steep debris‐flow prone channels prior to and following wildfire

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